The present invention generally relates to a hydrogen permeable structure and a method of manufacturing the same, and more particularly, to a hydrogen permeable structure in which a hydrogen permeable film is formed on a porous base material and a method of manufacturing the same.
Hydrogen gas is used as a fuel for a fuel cell and the like, and is manufactured e.g. by a method of transforming gaseous fuel. For instance, according to the method of transforming gaseous fuel, water vapor is reformed to produce hydrogen gas, the reformed gas including, in addition to hydrogen as a principal component, carbon monoxide, carbon dioxide and the like as sub components. If the reformed gas is used as it is for a fuel cell, the cell performance deteriorates. Thus, there is a need for removing sub components, i.e. components other than hydrogen, to refine the reformed gas in order to obtain a high purity hydrogen gas. One refining method utilizes a characteristic of a hydrogen permeable film that selectively allows only hydrogen to pass through the film. For use, the hydrogen permeable film is formed on a porous support or base material.
For instance, Japanese Patent Laying-Open No. 11-267477 has proposed a hydrogen permeable structure in which a hydrogen permeable film such as a Pd film, Nb film or the like having a thickness of approximately 0.1 to 20 xcexcm is formed by an ion plating technique on the surface of a porous support made of stainless steel or ceramic such as alumina and silicon nitride.
Moreover, Japanese Patent Laying-Open No. 11-286785 has proposed a hydrogen permeable structure in which Pd metal and metal to be alloyed with Pd are alternately layered on the surface of a porous support by an electroless plating technique or the ion plating technique, which is subsequently subjected to a heating process, to form a Pd alloy film as a hydrogen permeable film.
Furthermore, Japanese Patent Laying-Open No. 4-349926 has proposed a hydrogen gas separation film in which silica gel having an average pore diameter of 10 to 30 xc3x85, alumina gel having an average pore diameter of 15 to 30 xc3x85 or silica-alumina gel having an average pore diameter of 10 to 20 xc3x85 is formed in pores of an inorganic porous body having pore diameters in the range between 10 and 10000 xc3x85, and a thin film containing palladium is formed on the surface thereof as a hydrogen permeable film.
Japanese Patent Laying-Open No. 10-28850 has proposed a hydrogen separation structure including a base material made of porous ceramic or porous glass, a first layer layered on the base material, and a second layer layered on the first layer and made of Pd or a Pd alloy as a hydrogen permeable film, the first layer being formed of a material having a thermal expansion coefficient within the range between that of the base material and that of the second layer. The first layer relieves stress applied between the base material and the second layer when the hydrogen separation structure is exposed to an atmosphere with large temperature variation, to prevent the second layer from peeling off from the base material.
Japanese Patent Laying-Open No. 11-267477, Japanese Patent Laying-Open No. 11-286785, or Japanese Patent Laying-Open No. 4-349926 discloses a structure in which a hydrogen permeable film is formed on the surface of a porous support, which has suffered from peeling of the hydrogen permeable film when the hydrogen permeable structure is used in the atmosphere of various conditions, presenting a problem in durability.
To prevent the hydrogen permeable film from peeling off, the hydrogen separation structure disclosed in Japanese Patent Laying-Open No. 10-28850 has employed a layer, formed of a material having a thermal expansion coefficient within the range between that of a porous base material and that of a hydrogen permeable film, interposed between the porous base material and the hydrogen permeable film.
By merely relieving the difference in the thermal expansion coefficients between the porous base material and the hydrogen permeable film, however, it was difficult to effectively prevent peeling of the hydrogen permeable film.
An object of the present invention is, therefore, to provide a hydrogen permeable structure that can more effectively prevent peeling of a hydrogen permeable film and thereby having increased durability, and a method of manufacturing the same.
The present inventors have examined various possible causes of peeling of a hydrogen permeable film, and found that the primary cause of the peeling is the compressive stress occurring due to lattice expansions of metallic crystals associated with hydrogen dissolution, rather than the difference in thermal expansion coefficients between a porous base material and the hydrogen permeable film, and that such peeling can be prevented by forming a hydrogen permeable film with a small amount of hydrogen dissolution.
Based on such findings, the above object has been achieved according to the invention in a hydrogen permeable structure comprising a base made of a material including a porous ceramic, said base having a base surface with at least one pin hole in said base surface, a porous oxide material filling said at least one pin hole thereby making said base surface plane, a hydrogen permeable film including palladium and at least one element other than palladium on said plane base surface, said hydrogen permeable film having, at a prescribed temperature, an amount of hydrogen dissolution, which is smaller than an amount of hydrogen dissolution in palladium alone at said prescribed temperature.
The above defined hydrogen permeable structure is manufactured according to the invention by the following steps:
a) producing a base of a material including a porous ceramic material, said base having a base surface with at least one hole in said base surface,
b) filling said at least one hole in said base surface with a porous oxide material to thereby make said base surface plane,
c) forming on said plane base surface a hydrogen permeable film made of a film material including palladium and at least one element other than palladium, said film material having at a prescribed temperature, an amount of hydrogen dissolution that is smaller than an amount of hydrogen dissolution in palladium alone at said prescribed temperature. Preferably the forming step is performed by vapor deposition.
A modified embodiment of the present hydrogen permeable structure is manufactured by the following steps:
a) producing a base of a material including porous ceramic material having a base surface,
b) vapor depositing on said base surface a hydrogen permeable film made of a film material including palladium and at least one element other than palladium, said film material having, at a prescribed temperature, an amount of hydrogen dissolution that is smaller than an amount of hydrogen dissolution in palladium alone at said prescribed temperature, and
c) performing said step b) in a vacuum atmosphere of 13.3 Pa at the most, and by applying a potential difference of at least 400 V between said base and a raw material which provides said film material.
The amount of hydrogen dissolution in % by weight is defined as a value measured according to the method described in the EXPERIMENTAL section of xe2x80x9cSolubility of Hydrogen in Palladium-Silver Alloysxe2x80x9d in Russian Journal of Physical Chemistry 47(1) published in 1973, and is based on a value measured using a bulk sample with the same composition as the hydrogen permeable film.
According to the present invention the hydrogen permeable film has, at a prescribed temperature, a hydrogen dissolution amount that is, smaller than the hydrogen dissolution amount of palladium alone at the same temperature. Compared with a conventional hydrogen permeable metal film of palladium alone, in the working temperature range including a prescribed temperature, the invention reduces the hydrogen dissolution amount of the hydrogen permeable film thereby reducing the expansion of the crystal lattice of the palladium metal and of the film, whereby film peeling is suppressed. The compression stress imposed on the film by its expansion is reduced, whereby the stress applied at the interface between the film and the base is reduced. This feature significantly reduces the physical deterioration of the hydrogen permeable film particularly peeling, cracking and the like, are reduced and the durability of the hydrogen permeable structure is improved.
Preferably, the prescribed temperature in the hydrogen permeable structure, is at least 200xc2x0 C. and at most 700xc2x0 C.
More preferably, in the hydrogen permeable structure of the present invention, the at least one element other than palladium that is included in the hydrogen permeable film is platinum (Pt).
More preferably, in the hydrogen permeable structure of the present invention, the hydrogen permeable film includes palladium and platinum, the content of the platinum being at least 5% by mass and at most 15% by mass. Increasing the content of platinum can further reduce the amount of hydrogen dissolution into the film, though it lowers the permeability or permeation speed of hydrogen gas through the film. In order to improve the hydrogen gas permeability to a degree higher than that of the hydrogen permeable structure made of palladium alone and to enhance the durability of the hydrogen permeable structure by reducing the amount of hydrogen dissolution into the film, it is suggested that the content of platinum in the hydrogen permeable film, including palladium and platinum, is preferably set within the range between 5 to 15% by mass.
In the hydrogen permeable structure of the present invention, the porous ceramic forming the base is preferably silicon nitride (Si3N4). Among various types of ceramic, silicon nitride is superior in strength, fracture toughness, abrasion resistance, chemical resistance and heat resistance, whereby a further enhancement of the durability of the hydrogen permeable structure of the present invention is achieved.
The porous base material including porous ceramic material has at least one hole in the surface, and a porous oxide material or layer is preferably applied to fill the hole. Thus, the surface of the base material is made plane while the hole at the surface is filled with the porous oxide layer, allowing the hydrogen permeable film to be formed on the surface of the base material in a closely packed manner without pin holes, whereby the permeability of the hydrogen permeable film is improved. Moreover, the adhesion between the surface of the base and the hydrogen permeable film is enhanced, thereby further improving the durability of the hydrogen permeable structure. The porous oxide layer or material preferably includes at least one material selected from the group consisting of aluminum oxide (Al2O3), silicon dioxide (SiO2) and zirconium oxide (ZrQ2). The most preferred porous oxide material is aluminum oxide.
As described above, according to the present invention, peeling of a hydrogen permeable film and physical deterioration such as peeling and cracks have been significantly reduced, whereby the durability of the hydrogen permeable structure has been improved.